At the present time, a light-polarizing film is generally produced by the adsorption of a light polarizing element such as iodine or dichroic dyes into a stretched and oriented film made of polyvinyl alcohol or its derivatives, or into an oriented polyene film prepared by dehydrochlorinating a polyvinyl chloride film or dehydrating a polyvinyl alcohol film to form a polyene structure in these films.
Of these light-polarizing films, those in which iodine is used as a polarizing element are poor in resistance to moisture and heat although they are excellent in the initial polarization performance. Consequently, when they are used for a long period of time in high-temperature and high-humidity conditions, their durability becomes a problem. In order to improve the durability, there have been proposed various methods, for example a method of strengthening by treatment of these films with aqueous solutions containing formalin or boric acid, a method of using as a protecting film a high-polymer film which is low in permeability to moisture, and the like. However, the durability is still unsatisfactory in high-temperature and high humidity conditions.
Light-polarizing films in which a dichloric dye is used as a polarizing element are superior in the resistance to moisture and heat as compared with those in which iodine is used as a polarizing element. They are however inferior in the polarization performance as compared with the latter. Further, some of them change considerably in color under high temperature conditions depending upon the dyes used.
An object of the present invention is to solve the foregoing problems inherent to the prior arts, thereby providing a light-polarizing film which changes in color only with difficulty even under high temperature conditions and has a high performance.